If you're planning to buy the iPhone 5 and want to unload your existing phone, you've got a few hours left to lock in your best deal at sites like Gazelle.com before the new phone is announced.

See also: iPhone 5 review.

If you're planning to buy the iPhone 5 and want to unload your existing phone, you've got a few hours left to lock in your best deal at sites like Gazelle.com before the new phone is announced. See also: iPhone 5 review.

"A consumer can currently trade-in a 160GB iPhone 4S in good condition for up to $277," says Anthony Scarsella, Chief Gadget Officer for Gazelle, a consumer electronics trade-in site.

"In the past, we've seen prices begin to fall as quickly as one hour after the announcement, and these prices tend to steadily decline for about one month before leveling off. We're expecting to see the same thing after Apple makes its announcement [today]."

The new iPhone 5, being announced by Apple today at 1 p.m. ET, is expected to have the same pricing as the iPhone 4S introduced in October 2011. Assuming the same storage options, the iPhone 5 pricing, based on two-year cellular contract, will be: $199, for 16GB, $299 for 32GB, and $399 for 64GB.

Another option is a site like Glyde.com, which offers an online marketplace where you can offer your current phone (or other stuff) for sale. Used iPhone 4 models are currently being offered for about $270 and 4S models for about $370.

Following past practice, Apple then would cut the price of the iPhone 4S to $99 with a two-year contract.

It's complicated by the fact that carriers are offering buy-back programs as well as offering iPhone 4S at new low prices. Sprint, for example, will pay up to $300 for your current phone (not limited to iPhones) and currently offers a 16GB iPhone 4S, with contract, for $150; and a pre-owned 32GB 4S for $200.

Virgin Mobile, using Sprint's network, began offering in June both the iPhone 4 and 4S models at full price with its no-contract Beyond Talk unlimited data and messaging plans, starting at $30-per-month, with no activation or roaming fees. The16GB 4S is currently priced at $650 and the 8GB iPhone 4 is at $550, which seem likely to fall after today.

Price cuts in rival Android phones might entice you to opt for the green robot instead of the bitten apple. Amazon has slashed the price of the 16GB Samsung Galaxy S III for AT&T, Sprint and Verizon Wireless to $100 for a limited time.

To use Gazelle, you log on, select your make, model, its condition, and Gazelle quotes you a price. If you accept, Gazelle sends you an envelope, you send the device back in it, and Gazelle sends you back money, in the form of cash, PayPal, or other options, in three to five business days. If you don't like the deal, Gazelle promises to send your phone back, no questions asked.

Gazelle, which focuses heavily on Apple products, surveyed 2,700 online visitors regarding their buying plans: 83% are current iPhone users, 11% use Android phones. Of the total sample, 83% said they were planning to upgrade to the new iPhone.

When asked about what features on their current phone prompted the upgrade decision, 37% said lack of 4G/LTE support, 31% cited the phone's age, and 28% mentioned the small screen.